Elemental metal-metal bonding is a solid-state diffusion bonding process and needs relatively long bonding time at an elevated temperature. For example, Cu—Cu diffusion bonding process takes about 30 min at more than 300° C. Chip-on-wafer bonding can be performed by metal-metal diffusion bonding between the chip and wafer. In view of the time-consuming bonding process, flip chip bonders are rarely used for chip-on-wafer bonding to minimise the cost involved.
Conventionally, chip-on-wafer bonding involves solder-assisted bonding. Solder-assisted bonding are carried out in a relatively short period of time, e.g. about 20 seconds. Solders are first aligned and heated to its melting point, followed by a bonding process. Metal-metal bonding with flip chip bonder alone is also not practical since metal-metal bonding is a diffusion-dominant bonding process which takes relatively more time, usually over 30 minutes. Here, two step bonding process appears to be feasible. In particular, a chip is first aligned and temporarily bonded to a wafer using a flip chip bonder followed by a permanent bonding process carried out by a conventional bonder. In particular, a chip is first aligned and temporarily bonded to a wafer using a flip chip bonder followed by a permanent bonding process carried out by a conventional bonder. The chip and wafer can be temporarily bonded using an adhesive, or thermosonic (ultrasonic) bonding. The adhesive may remain between chip and wafer after permanent bonding, or may vaporize during the permanent bonding. The temporary bonding process is usually carried out below 100° C. Again, the bonding time per chip is relatively short in contrast with permanent bonding as it only involves pressing the chip onto the wafer. Depending on the required accuracy of the alignment, the required time for temporary bonding varies. For example, if the target alignment accuracy is 3 μm (3σ), total bonding time can be within 3 seconds.
Upon completion of the temporary bonding process of all the chips on wafer, the product of the temporary bonding process is placed on a bottom stage of the conventional bonder. Subsequently, a top stage presses and applies force on the chips. Bottom and top stages are then heated to a suitable temperature for permanent bonding.
It has been noted that chips tend to get misaligned during the permanent bonding process as described above. For example, top stages and surfaces thereof may be made of material, e.g. stainless steel that have different coefficient of thermal expansion (CTE) than that of wafer. Non-uniform expansion of the top stage and the wafer may result in a horizontal force exerted on the chips.
Further, chips which are temporarily bonded on the wafer can have slightly different thicknesses. As illustrated in FIG. 1, when the top stage 102 presses and applies force on the chips 104, it is likely that thicker chips receive the force while chips with lower thickness do not receive sufficient bonding force thereby resulting in a non-uniform bonding force and poor bonding between some of the chips and wafer.
With reference to FIG. 2, in order to solve the issues of non-uniform bonding force and poor bonding, a soft plate 202 that is made of deformable material is typically placed between the chips 204 and a topside stage 206. As such, chips with different height may still receive sufficient force during permanent bonding. However, the deformation of the soft plate around the chips may generate a horizontal force component on the neighbouring chips, causing chips to misalign with the wafer.
A need therefore exists to provide a method for chip-to-wafer integration that seeks to address at least some of the problems above or to provide a useful alternative.